A majority of patients who require prescription lenses now prefer and purchase plastic lenses. Plastic lenses (otherwise known as ‘organic glass’ lenses) have numerous advantages over the more traditional glass lenses, including reduced weight, higher clarity, they can be dyed or tinted easily, machined easily and they are relatively stable.
Plastic lenses are typically made by cast moulding, in which a monomer or prepolymer material is deposited in a cavity defined between optical surfaces of opposing mould parts. The monomer or prepolymer material is then cured in the assembled mould to form a solid plastic lens. Both thermoplastic and thermoset resins are used for this purpose. Particular thermoset resins that have been used include polymers of diethylene glycol bis(allyl carbonate) such as CR-39™ (a trade mark of PPG Industries), whilst a popular thermoplastic resin that is used is a polycarbonate of bisphenol A.
One problem associated with some of the materials used to form plastic lenses is their relatively low refractive index. Low refractive index plastic lens materials may be less desirable than other materials because of the thicker lens optic necessary to achieve a given refractive power.
To address the problems with the refractive index of these materials a range of thiourethane-based plastic lens materials have been developed. These materials typically have a relatively high refractive index which means that thinner lenses can be formed for a given refractive power. Considerable research has now been carried out with sulfur-containing polyurethane compositions in order to maximise the refractive index of the lens materials and/or improve other physical or optical properties of the materials whilst maintaining an acceptably high refractive index.
Thiourethane-based plastic lenses can be obtained by reacting a polyisocyanate monomer or prepolymer with a polythiol monomer or prepolymer. The prior art discloses a number of lens resin compositions that can be polymerised to form a thiourethane polymer. For example, U.S. Pat. Nos. 4,680,369, 4,689,387, and 4,780,522 disclose lens resin compositions for forming high index (ND20° C. 1.59 or higher) lenses. The compositions contain an aromatic diisocyanate and sulfur containing compounds.
Attempts have also been made to improve other properties of thiourethane lenses. For example, U.S. Pat. No. 4,791,185 discloses lens resin compositions for forming thiourethane lenses having heat resistance to withstand heating conditions in post lens formation processing. The compositions contain a diisocyanate and an isocyanurate-based thiol. Similarly, U.S. Pat. No. 5,059,673 discloses a lens resin composition for forming heat and weather resistant urethane or thiourethane lenses. The lenses are formed by reacting specific polycyclic alkane isocyanates with a polyol compound, a polythiol compound or a thiol compound having a hydroxyl group.
U.S. Pat. No. 5,084,545 discloses a lens resin composition for forming high index urethane or thiourethane lenses. The lens is formed by reacting an isothiocyanate compound with a polyol compound, a polythiol compound or a thiol compound having a hydroxyl group.
There have also been attempts to improve the processing conditions used to form urethane or thiourethane lenses. For example, U.S. Pat. Nos. 5,087,758 and 5,191,055 disclose a mercpato compound having an insensible sulfurous odour and a polythiourethane resin containing the mercapto compound. The mercapto compound is reacted with a polyisocyanate compound, a polyisothiocyanate compound or an isothiocyanate compound having isocyanato groups to form the polythiourethane resin.
U.S. Pat. No. 5,440,358 discloses a high surface hardness lens that is formed by reacting a urethane or thiourethane based monomer having a polymerisable double bond with an acrylate, methacrylate or vinyl monomer.
U.S. Pat. No. 5,310,847 discloses lens resin compositions for forming polyurethane lenses having high refractive index (ND20° C. higher than 1.6) and a glass transition temperature higher than 100° C. The lenses are formed by reacting an at least difunctional polyisocyanate with an acyclic saturated monomer having at least three reactive groups with respect to isocyanates per molecule and having at least 40% thiol groups.
U.S. Pat. No. 5,635,580 discloses a process for the production of a thiourethane lens which serves to decrease the polymerisation time. The process comprises reacting a polyisocyanate compound with two or more polythiol compounds which have different reaction rates with the polyisocyanate compound to form a thiourethane lens.
From the aforementioned description it is clear that by altering the specific monomers used and/or the processing conditions it is possible to form thiourethane lens materials having specific, tailored optical or physical properties. Despite the considerable research that has been carried out to date, there are still drawbacks associated with some of the prior art thiourethane lens materials described above. For instance, technical and economic difficulties may be encountered because the monomers used are not readily available or they are viscous solutions which makes casting more complex. Furthermore, the impact resistance of some of the thiourethanes formed may not be sufficient to allow for the formation of relatively thin lenses. Also, some of the thiourethanes formed have a glass transition temperature that is too low with respect to temperatures necessary for the surface treatment of optical lenses.
Clearly, there is a need for improved and/or alternative materials or processes for forming thiourethane optical lens elements.
Reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in any country.